The invention relates to a ventilation-mast monitoring system for filling stations.
At filling stations, the fuels which are intended for refuelling motor vehicles are generally stored in reservoir tanks which are buried in the ground. Such a reservoir tank is connected to a ventilation mast which projects out of the ground and by means of which, depending on the pressure conditions prevailing in the reservoir tank, gas (in particular a fuel/air mixture) can escape from the reservoir tank or air can enter the reservoir tank. The pressure in the reservoir tank can vary, for example if the fuel cools to the temperature of the ground after the reservoir tank has been filled. Also, pressure fluctuations occur in the reservoir tank if, when refuelling a motor vehicle, the fuel feed rate does not correspond to the gas feed rate of the gas recirculation system. The cause of this may be, for example, faults in the gas recirculation system or refuelling processes in motor vehicles in which fuel vapours are retained with on-board means (ORVR). Since the pressure in a reservoir tank can increase and decrease, and as little fuel gas or vapour as possible should be allowed to escape into the environment, ventilation masts are frequently provided in their upper end region with a throttle or a gas pendulum valve. A throttle has a high flow resistance and therefore reduces the gas volume flow through the ventilation mast while a gas pendulum valve acts as an overpressure valve in both directions so that gas can flow through the ventilation mast only if an overpressure in the reservoir tank exceeds a predefined value or an underpressure drops below a predefined value.
A ventilation-mast monitoring system can be used to acquire an overview of the pressure conditions in a reservoir tank of a filling station and, if appropriate, to adjust the pressure. Such a system is described in EP 0 985 634 B1. In said system, the ventilation mast is provided with a gas pendulum valve, a non-return valve, a through-flow meter and a mass spectrometer serving as a hydrocarbon sensor. The measured data is processed in a controller and makes it possible, in particular, to recognize an ORVR vehicle when refuelling and to set the gas recirculation accordingly.
The through-flow meter of the previously known ventilation-mast monitoring system is a conventional device which is limited in its measurement dynamics and, for example, can no longer sense quantitatively if a large quantity of gas escapes through the ventilation mast while the reservoir tank is being filled, because the gas pendulum hose, which serves to recirculate the gas expelled out of the reservoir tank during refuelling into the tanker vehicle, has inadvertently not been connected.